Jack and I getting sticks out as shield, united in self-defence (7)
I believe the answer is:
jujitsu
'self-defence' is the definition.
(jujitsu is a kind of self-defence)
'jack and i getting sticks out as shield united' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'and' is a charade indicator (letters next to each other).
'getting sticks out' becomes 'juts' (I am not sure about this - if you are sure you should believe this answer much more).
'as shield' indicates putting letters inside.
'united' becomes 'u'.
'j'+'i'='ji'
'ji' inserted inside 'juts' is 'jujits'.
'jujits'+'u'='JUJITSU'
'in' acts as a link.
(Other definitions for jujitsu that I've seen before include "Self-defence system" , "Japanese method of unarmed self-defence" , "Samurai self-defence" , "Japanese martial art" , "Eastern method of self-defence without weapons" .)